Metallic nanoparticles composed of gold and silver may possess desired chemical, electronic, and optical properties, including an ability for size-controlled synthesis, stabilization, functionalization, and bio-compatibility. Due to these properties, metallic nanoparticles may allow for desired applications in the fields of molecular sensing, catalysis, photothermal therapy, and biologically-relevant technologies, such as bio-imaging and bio-sensing. For example, photothermal therapy may provide a non-invasive approach that can have fewer side effects than conventional treatment, such as chemotherapy and/or radiation therapy. A variety of plasmonic metallic nanoparticles may have potential use in photothermal therapy, including gold nanorods, silver nanorods, gold nanocages, silica-gold core-shell, and gold nanoparticles coated with reduced graphene oxide. Photothermal therapy may utilize plasmonic nanoparticles with near-infrared wavelength absorption in the ranging from about 800 nm to about 1300 nm. Due to the plasmonic enhancement of the metallic portions of the nanoparticle, these nanoparticles can absorb light in the near-infrared wavelengths, corresponding to the optical window in biological tissues. The nanoparticles can convert the absorbed light into heat through a nonradiative process leading to a localized photothermal effect that can be used for photothermal therapy, and/or non-invasive bio-imaging due to their tunable optical properties and their biocompatibility.
Metallic nanoparticles may be difficult to synthesize and keep stable in colloidal suspensions, and some inherent properties of the metals or other materials used in the nanoparticles may present limitations. As an example, spherical gold nanoparticles may possess a very low light absorbance at near-infrared wavelengths, so they may not be preferred for potential biological applications, such as photothermal therapy and diagnostics. In another example, the thickness of the nanoparticle, which can affect its properties and potential applications, may be difficult to control in processing. In yet another example, if a nanoparticle comprises a non-metallic portion, such as gold-coated silica nanoparticles, the resulting particle may have a smaller plasmonic enhancement property than metallic nanoparticles, due to their non-metallic core. Although silica-based plasmonic nanoparticles may have desirable light absorbance at near-infrared wavelengths, their non-fully metallic composition can reduce their conversion of light to heat efficiency, which can make them less effective in killing cancer cells. In another example, both gold-reduced graphene oxide nanoparticles and gold nanorods may absorb at the near-infrared wavelength window. However, synthesis of a uniform reduced graphene oxide shell can be difficult, reproducibility of the gold nanorods may be difficult, and the gold nanorods can be unstable over long period of times.